Neoadjuvant Chemoradiotherapy Combined With PD-1 Inhibitor and Thymalfasin for pMMR/MSS Locally Advanced Mid-low Rectal Cancer

Inclusion Criteria: Patients who were fully informed of the study and voluntarily signed the informed consent form; Patients who were pathologically diagnosed with rectal adenocarcinoma via enteroscopic biopsy and met the following criteria: ①LARC at clinical stage II/III: cT1-4aN0-2M0; ② Distance between distal tumor and the anal verge ≤10 cm (measured by MRI); ③ No distant metastasis; ④ pMMR or MSS confirmed by immunohistochemistry or genetic test. Men and women aged ≥18 and ≤75 years old, with ECOG score of 0-1 points; Patients willing to accept the treatment protocol of neoadjuvant therapy + TME; Patients whose major organ functions met the following requirements (The values of laboratory tests within 14 days before enrollment must meet the following criteria): ① Blood routine test: (no blood transfusion or use of granulocyte colony-stimulating factor or drug correction within 14 days before screening): a) neutrophil count ≥1.5 × 109/L; b) platelet count ≥75 × 109/L; c) hemoglobin level ≥90 g/L ② Biochemical test: (no albumin infusion within 14 days prior to screening): a) serum creatinine ≤1.5 × upper limit of normal (ULN) or creatinine clearance >50 mL/min; b) serum total bilirubin ≤1.5 × ULN; c) aspartate aminotransferase (AST), alanine aminotransferase (ALT) ≤2.5 × ULN ③ Coagulation function: a) international normalized ratio (INR) ≤2.3 or prothrombin time (PT) ≤6 seconds longer than the range of normal control; ④ Normal thyroid function No immune system diseases (e.g., systemic lupus erythematosus, rheumatoid arthritis (RA), systemic vasculitis, scleroderma, mixed connective tissue disease, autoimmune hemolytic anemia, hyperthyroidism, hypothyroidism, and ulcerative colitis); Within 28 days before enrollment, women of childbearing age must have a negative serum pregnancy test and agree to take effective contraceptive measures during the use of investigational drug and within 60 days after the last drug administration. Female spouses of male subjects should also follow the above contraceptive requirements if they are of childbearing age; Patients whose body and organ functions can tolerate major abdominal surgery. Exclusion Criteria: Patients currently or previously with malignant tumors other than rectal cancer (except for cured basal cell carcinoma of skin or cervical carcinoma in situ); Patients treated with major surgery within 4 weeks before the start of research therapies; Patients with any condition that affected the absorption of capecitabine in the gastrointestinal tract; Patients with severe uncontrolled recurrent infections, or other severe uncontrolled concomitant diseases; Patients with severe diseases with an estimated survival of ≤5 years; Patients with congenital malformation, developmental disorder, genetic defect, or severe malnutrition; Patients who were ready to receive or had received organ or bone marrow transplantation; Pregnant or lactating women; Patients with severe cardiovascular disease, hepatic or renal disease, respiratory disease, uncontrolled hypertension (systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg), diabetic complication, various acute diseases or acute stage of chronic diseases; Patients diagnosed with congenital or acquired immune deficiency (such as HIV infection), lymphoma, leukemia, or other autoimmune diseases, or those suffering active tuberculosis or receiving anti-tuberculous therapy; Patients who received immunotherapy within 3 months before screening, such as long-term treatment with prednisone at a dose >10 mg/day or corticosteroid at an equivalent dose (more than 14 consecutive days through oral administration or infusion), or immunosuppressant; Patients who received blood or blood-related products, including immunoglobulin, within 3 months, or those who planned to use them at the beginning of the study; Patients treated with other investigational drugs within 1 month before screening; Patients with a history of allergy or intolerance to any investigational drug or its components; Patients with suspected or known alcohol or drug dependence; Patients with dMMR or MSI-H confirmed by immunohistochemistry or genetic test; Patients with T4b or MRF(+) according to baseline rectal MRI; Patients with other conditions unsuitable for this clinical trial judged by the investigator.